1. Field of the Invention
The field of the invention is that of compact optical sources capable of emitting in the spectral domains, in which emission is not possible for lack of appropriate laser materials or the difficulty of obtaining them, these sources carrying out their emission by operations of frequency conversion.
Indeed, by using the second-order non-linear susceptibility of certain materials such as lithium niobate (LiNbO.sub.3), it is possible, with an illumination in the near infrared, to emit in the blue region of the spectrum through the phenomenon of frequency doubling.
In the frequency doubling operation, in order that the optical power transfer done on the basis of the incident illumination at the wavelength .lambda. may be efficient, it is necessary, in the material and throughout the interaction, for the non-linear polarization induced by the incident wave to have its phase matched with the wave created at the wavelength .lambda./2 which it is sought to feed. However, in general, and because of the dispersal of the refraction indices of the material at .lambda..sup..omega. and .lambda..sup..omega.b /2, this phase-matching condition is not met straight away.
2. Description of the Prior Art
To circumvent this obstacle, a first approach consists in taking advantage of the birefringence of certain materials in order to compensate for the range of variation of their index. There also exists an alternative approach that consists of the use of quasi-phase matching applicable in conditions that are far more general than those relating to the birefringence of certain materials. This alternative approach consists in periodically disturbing the non-linear interaction in order to compensate for the wave vector difference responsible for the phase mismatching. More precisely, let us take an incident wave being propagated along the axis Ox in the non-linear material and having electrical fields: EQU E.omega.=E.omega.,oe.sup.j(.omega.t-.beta..sbsp..omega..sup.x+.phi.p.sbsp.o .sup.)
with a wavelength .lambda..omega.=2 .pi..c/.omega. with .beta..sub..omega. being the constant of propagation of the pump wave
and .beta..sub..omega. =2 .pi.n.sub..omega. /.lambda..sub..omega. PA1 c: the velocity of light in vacuum PA1 .omega.: frequency. PA1 the phase matching condition enabling the cancellation of or compensation for the difference in propagation constant between firstly the non-linear polarization created by the incident wave and, secondly, the harmonic wave generated by this polarization is achieved at the wavelength .lambda.o.sub.a belonging to the set of .lambda.o.sub.i values, in the medium (NLM); PA1 the optical source also includes two dichroic mirrors M.sub.1 and M.sub.2 placed in such a way that there is the following succession of elements: the emitter laser, the mirror M.sub.1, the medium (NLM), the mirror M.sub.2, said mirrors M.sub.1 and M.sub.2 being highly transparent at the wavelengths .lambda.o.sub.i and highly reflective at the wavelengths .lambda.o.sub.i /2 so as to reinject a wave at .lambda.o.sub.a into the laser and recover at M.sub.1, a wave at the wavelength .lambda.o.sub.a /2.
In an appropriate material, this wave may give rise to a second-order non-linear polarization written as follows: EQU P.sub.NL =.epsilon..sub.o dE.sub..omega..sup.2 =.epsilon..sub.o dE.sub..omega.,o.sup.2 e.sup.j(2.omega.t-2.beta..sbsp..omega..sup.x+2.phi..sbsp.po.sup.)
where d is the non-linear coefficient brought into play and.epsilon..sub.o is the dielectric permittivity of the vacuum.
This polarization radiates a wave at double frequency liable, as and when the interaction takes place, to set up a harmonic beam with a half wavelength .lambda..sup.2.omega. =.lambda..sup..omega./2 and a propagation constant .beta..sub.2.omega. with .beta..sub.2.omega. =2 .pi.n.sub.2.omega. /.lambda..sup.2.omega. where n.sub.2.omega. is the refraction index of the material at the wavelength .lambda..sup.2.omega.. The electrical field corresponding to this wave may be written as follows: EQU E.sub.2.omega. =E.sub.2.omega.,oe.sup.j(2.sbsp..omega.t.sup.-.beta..sbsp.2.omega..sup.x+. phi..sbsp.ho.sup.)
It can thus be seen that the phase shift .DELTA..phi. between the non-linear polarization that forms the source of the radiation at .lambda..sup.2.omega. and the harmonic wave that is to be fed constructively by means of this polarization will play a decisive role in the conversion .lambda..sup..omega. .fwdarw..lambda..sup.2.omega.. In fact, this phase shift is expressed at the end of a distance x of interaction as follows: EQU .DELTA..phi.=(.beta..sub.2.omega. -2.beta..sub..omega.)x=.DELTA..beta.x
with EQU .DELTA..beta.=4 .pi.(n.sub.2.omega. -n.sub..omega.).lambda..sup..omega. =4 .pi..DELTA.n/.lambda..sup..omega.
It can clearly be seen that, because of the range of variation of the indices, this phase shift is generally not zero.
However, it is possible to create a periodic variation .DELTA..beta.=m.K or K=2 .pi./.LAMBDA. with .LAMBDA. as the period of the disturbance and .LAMBDA.=2L.sub.C if L.sub.C is the length of coherence corresponding to the interaction distance at the end of which the polarization and harmonic wave have accumulated a .pi. phase shift.
The disturbance may be introduced at any parameter coming into play in the non-linear interaction (refraction index, dispersion of the indices, non-linear coefficient brought into play, etc.).
Through this periodic disturbance, the phase shift .DELTA..phi. between polarization and harmonic wave is reduced by .pi. at the end of each L.sub.C, namely instead of continuing to accumulate, it is reduced to zero at each coherence length. In this respect, FIG. 1 illustrates the three possible examples: curve a) phase mismatching, curve b): quasi-phase matching, curve c): perfect phase matching.
The object of the invention relates to a source using a laser emitting an instant wave at .lambda..sup..omega. so as to generate a wave .lambda..sup.2.omega. by means of a frequency doubler, the frequency doubler being a non-linear medium (NLM) in which the phase matching condition or the quasi-phase matching condition is achieved at the wavelength .lambda..sup..omega..
The source according to the invention enables the problem of the spectral width of emission of the laser to be resolved. This phenomenon is pronounced in the laser diode context whereas the phase matching conditions or quasi-phase matching conditions are met in the medium (NLM) for certain very precise wavelengths only. For this purpose, the compact source according to the invention uses the "locking" of the emission length of the laser by the injection, into this wave, of a signal with a wavelength identical to that for which the phase matching condition or quasi-phase matching condition is ensured.
This notion of "locking", which corresponds to the fact of injecting an optical signal into a laser and notably a laser diode in order to lock the emission wavelength of this diode to the wavelength of the injected signal, has already been dealt with in published work. Thus, the authors of an article (K. Yamamoto, H. Mizuuchi, Y. Kitaoka and M. Kato, "High power blue light generation by frequency doubling of a laser diode in a periodically domain-inserted LiTaO.sub.3 waveguide, Appl. Phys. Lett. 62 (21), 24 May 1993) [1] have achieved the continuous variation of the emission wavelength of a monofrequency laser diode by the reinjection, into this diode, of its own beam but after reflection on a dispersive grating (the diode not necessarily having undergone any anti-reflection treatment on the exit face). The schematic diagram of this experiment designed for the frequency doubling of a laser diode by quasi-phase matching (QPM) in a non-linear waveguide is shown in FIG. 2. The rotation of the grating enables the variation of the wavelength reinjected into the laser diode and hence makes it possible to check the wavelength of emission of this diode (in this case around 860 nm) to obtain the desired matching.
The major drawback of this type of technique lies in the fact that the desired matching is not obtained automatically and therefore that an additional feedback is needed so that the system works satisfactorily. In the example referred to here above, the position of the grating has to be servo-linked with the harmonic power emitted in order to follow a potential variation of the QPM wavelength of the doubler, for example under the effect of the external temperature.
The object of the invention is to enable the automatic locking of the wavelength of the laser diode into the wavelength for which the phase matching condition or quasi-phase matching condition is met at the frequency doubler.
For this purpose, the principle of the invention consists of the recycling, in the non-linear component (NLM) used for the frequency doubling, of the major part of the harmonic wave accompanied by a small part of the residual pump wave (the incident wave that generates the non-linear polarization).
It is indeed possible, in the non-linear component, to obtain the amplification of the small part of the recycled pump wave but this is done solely if its wavelength corresponds to that of the phase matching condition or QPM condition in the non-linear component. By reinjecting the optical signal from this amplification, the wavelength of the diode is locked at the exact value that enables the highest efficiency of conversion between the power from the laser diode and the harmonic power that corresponds to the output power of the proposed source.